The present invention relates to an optical storage medium, in which data is written and from which data is reproduced, using a laser beam. Specifically, the present invention relates to a write-once-type optical storage medium conforming to digital video disc (DVD) specifications. More specifically, the present invention relates to an optical storage medium that uses a highly sensitive and highly reliable organic dye thin film. High density data storage and data reproduction is achieved. The optical reflectance of the optical storage medium of the present invention, changes with irradiation of a writing light beam having a small beam diameter, making it particularly suited for high density data storage and for data reproduction.
Recordable optical storage media are in widespread use today. The optical storage medium is kept free from deterioration due to wear since the writing and reproducing head does not make contact with the storage medium. A large amount of data is stored in an optical storage medium by minimizing the diameter of the light beam for writing data. Many optical storage media have been developed and used for high-capacity data storage having these distinctive features. In making the optical storage media, a laser beam is focused onto a part of the storage layer of the optical storage medium, where the energy of the laser beam is converted to thermal energy. The properties and shape of the storage layer are changed by using thermal energy to melt, decompose and remove the irradiated part of the storage layer. As a result of this process, memory pits 6 which store the data are formed, as illustrated in FIGS. 1 through 3. The stored data is reproduced using the difference of the light quantities reflected between the portion where the data is stored and the portion where data is not stored.
At first, layers of tellurium (Te) and other such chalcogenide metals were developed for the storage layer. However, the chalcogenide metals are harmful to the human body. In addition, the dry method for forming the chalcogenide layers results in increased manufacturing costs. To avoid these disadvantages, and also to obtain high-density data storage, use of an organic dye as a main component of a storage layer in the storage media has been proposed. Although the optical reflectance of the organic dye storage layer is lower than that of the metal chalcogenide storage layer, the organic dye storage layer exhibits many merits. For instance, forming the organic dye storage layer by a wet method such as spin-coating results in lower manufacturing costs. In addition, the organic dye storage layer exhibits excellent resistance against acid and erosion. The organic dye storage layer facilitates local heating, which further facilitating the formation of clear and sharp memory pits. This effect is a result of the thermal conductivity of the organic dye storage layer being less than that of the chalcogenide metal storage layer.
Many structures for storage media have been proposed including the so-called air-sandwich structure which consists of a storage layer containing a conventional dye and an air layer on the storage layer. The structures thus formed facilitate obtaining signals reproduced in conformity with the compact disc (CD) specifications. These structures are described in Japanese Examined Patent Application No. H03-75943, Japanese Unexamined Laid Open Patent Applications No. H02-87341 and No. H05-67352, and Nikkei Electronics, No. 469 (Jan. 23., 1989), p107.
In a typical medium structure in conformity with the CD specifications, a light absorption layer containing an organic dye is formed on an optically transparent resin substrate. Hereinafter, the light absorption layer is sometimes referred to as the xe2x80x9corganic dye layer.xe2x80x9d A light reflection layer, including for example Au, is formed directly onto the light absorption layer. Alternatively, a light reflection layer is formed indirectly above the light absorption layer with a hard layer interposed in between. A resin protection layer is formed on the light reflection layer. The light reflection layer is indispensable for providing the surface of the organic dye layer with the optical reflectance of 65% or more required to meet the CD specifications.
When a laser beam irradiates the optical storage media, the organic dye layer absorbs the laser beam and is melted or decomposed, and the substrate is softened. The dye and the substrate mix at the boundary between the organic dye layer and the substrate. As a result, the boundary is deformed, creating memory pits. The reflectance of the thus formed pit portions changes with the optical phase difference in the same way as in the CD""s. The data is read out based on the reflectance change. The squalane dyes (disclosed in Japanese Unexamined Laid Open Patent Applications No. S56-46221, No. S63-218398, No. H01-178494, No. H05-139047 and No. H07-44904), the naphthoquinone dyes (disclosed in Japanese Unexamined Laid Open Patent Applications No. S61-290092, No. S62-432, No. S63-168201 and No. H05-139047), the azo dyes (disclosed in Japanese Unexamined Laid Open Patent Applications No. H07-161069, No. H07-251567 and No. H08-99467), the phthalocyanine dyes (disclosed in Japanese Unexamined Laid Open Patent Applications No. S57-82094, No. S57-82095, No. H07-156550, No. H07-16068, and No. H07-52544) and the cyanine dyes, having the general formula (III) described in FIG. 6, (disclosed in Japanese Unexamined Laid Open Patent Applications No. S59-24692, No. H02-87341, No. H06-320869, No. H06-338059, No. H06-199045, No. H07-262611 and No. S62-201288 and Japanese Examined Patent Application No. H07-4981) are used as the organic dye in the light absorption layer. The general formula (III) of the foregoing cyanine dyes is described in FIG. 6, where R15 represents an alkyl group, an aryl group or an alkoxy group; R16 an alkyl group, an aryl group or an alkoxy group; Y3 a halogen atom, a hydrogen atom or a substituent such as an alkyl group, an alkoxy group, an aryl group, an alkoxysulfonyl group, a sulfonylalkyl group and a cyano group, and Y4 a halogen atom, a hydrogen atom or a substituent such as an alkyl group, an alkoxy group, an aryl group, an alkoxysulfonyl group, a sulfonylalkyl group and a cyano group; Q1 a sulfur atom, an oxygen atom, a selenium atom or a substituent such as an ethylene group; and Q2 a sulfur atom, an oxygen atom, a selenium atom or a substituent such as an ethylene group.
Among the dyes described above, the cyanine dyes are primarily used due to their advantages including high sensitivity, high C/N ratio, excellent thermal properties and ease of layer formation. The cyanine dyes exhibiting high absorbance and reflectance in the wavelength from a semiconductor laser emitting a laser beam between 780 and 830 nm, are preferred for meeting CD specifications. One of the factors that determines the absorption wavelength of a molecule is the length of its xcfx80 conjugated system. In the cyanine dye molecule, the xcfx80 conjugated system absorbs light in the wavelength range between 780 and 830 nm. In the foregoing general formula (III), illustrated in FIG. 6, P represents the length of the ethylene chain in the central part of cyanine dye molecule. The value of P is generally 2. However, a dye as described in general formula (III) above, is not effective when a laser beam having a shorter wavelength is used to increase the storage density. Problems still remain concerning the deterioration of the optical storage media due to repeated reproduction, long term stability of the dye layer and C/N ratio.
A reduction of the C/N ratio and increase of the jitter components are caused by (i) deterioration and discoloration of the dye from heat accumulated in the storage layer due to prolonged exposure to the reading light, (ii) gradual melting and thermal deformation of the storage layer caused by the reading light on the boundaries for distinguishing the memory-pit portions from the non-memory-pit portions, (iii) oxidative deterioration (discoloration) of the dye by singlet oxygen generated by the energy transfer from the dye to oxygen in the environment while the dye is optically excited and (iv) a change in transparency of the cyanine dye caused by either natural light and oxygen contained in the dye or change of the transparency of the dye and noise caused by the association and aggregation of the dye molecules. This association and aggregation of the dye molecules is caused by exposure to oxygen and water molecules when the optical storage medium is stored for a long time. Although various proposals for solving the above described problems have been disclosed in Japanese Unexamined Laid Open Patent Applications No. S62-201288, No. S62-201289, No. S57-66541, No. S59-124894, No. S59-203247, No. S62-133173, No. S63-198096, No. S59-21339, No. S57-11090, No. S60-44389, No. S60-71296, No. S63-1594, No. H05-38879 and No. H07-262611 and Japanese Examined Patent Application No. H07-4981, the problems have not yet been solved.
Optical storage media have been developed for high density data storage which conform to the DVD-ROM specifications. These optical storage media increases the data storage density by using a beam from a semiconductor laser having a wavelength between 600 and 680 nm. This is shorter than the wavelength of the conventional laser beam for CD""s. In addition, the optical storage media conforming to the DVD-ROM specifications use a smaller diameter of beam spot. Improvements of the cyanine dyes for CD""s described by the general formula (III) illustrated in FIG. 6 have been made, and cyanine dyes for use at a shorter wavelength have been proposed (cf. Japanese Unexamined Laid Open Patent Applications No. H06-199045, No. H07-186530, No. H08-306074, No. H05-38879 and No. H06-40162).
Many problems remain unsolved when using organic dye layers in optical storage media, especially for DVD-ROM applications. For example, the sensitivity and the stability (reliability) of the organic dye layers developed so far, are not high enough at the desired wavelength. The S/N ratio and the C/N ratio are undesirably lower due to thermal interference between the adjacent pits in the high density data storage. Further, jitter in the reproducing signal is increased.
In view of the foregoing, it is an object of the invention to provide an optical storage medium that is compatible with a beam from a semiconductor laser having a short wavelength (i.e. between 500 and 700 nm).
It is another object of the invention to provide an optical storage medium that includes a highly stable dye layer.
It is still another object of the invention to provide an optical storage medium that facilitates reducing jitter components in high density data storage.
It is a further object of the invention to provide an optical storage medium that is in conformity with the DVD specifications.
Briefly stated, the present invention provides an optical storage medium including an optically transparent substrate having at least one major surface on which at least one groove is formed, a storage layer on the substrate and a metal reflection layer on the storage layer. The storage layer contains a composite consisting of from about 3 weight % to about 30 weight % of a metal complex compound and a cyanine dye. The cyanine dye has an asymmetrical structure and absorbs light in the wavelength between 500 and 700 nm.